The present invention relates to a gain controllable broadband amplifier. More particularly, the present invention relates to a broadband amplifier of the type which includes four differential amplifiers, each having two input ports, two output ports, and a shared port. The broadband amplifier itself has two signal input ports, two control voltage input ports, and two power supply ports. The two input ports of the first differential amplifier are connected to the two signal input ports of the broadband amplifier. The shared port of the first differential amplifier is connected, by way of a constant current source, with one of the power supply ports of the broadband amplifier. One output port of the first differential amplifier is connected to the shared port of the second differential amplifier and the other output port of the first differential amplifier is connected to the shared port of the third differential amplifier. One input port of the second differential amplifier is connected to one input port of the third differential amplifier, and both are connected to one of the control voltage input ports of the broadband amplifier. Furthermore, one of the output ports of the second differential amplifier is connected to one of the output ports of the third differential amplifier, and both are connected to the shared port of the fourth differential amplifier. The remaining output ports of the second and third differential amplifiers are respectively connected to the signal output ports of the broadband amplifier via connecting points and to the remaining power supply port of the broadband amplifier via a signal feed-out stage. Finally, in a conventional broadband amplifier of this type, the two input ports of the fourth differential amplifier are connected together and to a further voltage source.
Such a broadband amplifier is disclosed, for example, in Electronics Letters, Vol. 22, No. 8 (1986), pages 415-516. In contrast to the previously employed amplifier arrangements, in this prior art broadband amplifier the fourth differential amplifier reduces DC voltage fluctuations occurring at the amplifier output during gain control. However, even after this stabilization measure there remains at the output a DC voltage fluctuation of about 12 mV over a control range of 20 dB.